Various types of biometric systems are used more and more in order to provide for increased security and/or enhanced user convenience.
In particular, fingerprint sensing systems have been adopted in, for example, consumer electronic devices, thanks to their small form factor, high performance and user acceptance.
Among the various available fingerprint sensing principles (such as capacitive, optical, thermal etc.), capacitive sensing is most commonly used, in particular in applications where size and power consumption are important issues.
All capacitive fingerprint sensors provide a measure indicative of the capacitance between each of several sensing structures and a finger placed on or moved across the surface of the fingerprint sensor.
Some capacitive fingerprint sensors passively read out the capacitance between the sensing structures and the finger. This, however, requires a relatively large capacitance between sensing structure and finger. Therefore such passive capacitive sensors are typically provided with a very thin protective layer covering the sensing structures, which makes such sensors rather sensitive to scratching and/or ESD (electro-static discharge).
U.S. Pat. No. 7,864,992 discloses a capacitive fingerprint sensing system in which a driving signal is injected into the finger by pulsing a conductive structure arranged in the vicinity of the sensor array and measuring the resulting change of the charge carried by the sensing structures in the sensor array.
This type of so-called active capacitive fingerprint sensing systems generally enable measurement of the capacitance between finger and sensing structures with a much higher signal-to-noise ratio than the above-mentioned passive systems. This, in turn, allows for a considerably thicker protective coating and thus for more robust capacitive fingerprint sensors that can be included in items subjected to considerable wear, such as mobile phones.
For even further increased robustness and ease of integration of the fingerprint sensor into electronic devices etc., it is, however, desirable to enable fingerprint measurement through a very thick protective coating, which may be several hundreds of microns thick. For instance, it may be desirable to enable fingerprint measurement through a sheet of glass or similar, such as the front or back glass cover of a mobile phone.
When measuring through such a thick protective coating, however, the capacitive coupling between the finger and different sensing structures will be almost the same, making it very difficult to extract fingerprint information from the fingerprint sensor output. In other words, the portion of the input signal to the sensing structures carrying fingerprint information will be very small compared to the common input signal to the sensing structures.
To mitigate this problem, U.S. Pat. No. 8,888,004 proposes to apply an inverted version of the excitation signal or of the average of the received signal across the array of finger sensing pixels equally to the array of finger sensing pixels. In one implementation, U.S. Pat. No. 8,888,004 proposes to generate the average signal by interconnecting a group of finger sensing pixels that may not currently be scanned and measuring the average signal developed on this group of interconnected pixels.
Although the approach proposed by U.S. Pat. No. 8,888,004 may be potentially useful for suppressing the common signal, the suggested solution is expected to be rater complex and difficult to implement in practice.